Amplitude shift keying (ASK) and frequency shift keying (FSK) demodulation circuits are units configured to demodulate received signals in a digital microwave system receiving circuit, and are usually provided in an indoor unit of the digital microwave system. The indoor unit is usually connected with an outdoor unit through an intermediate frequency cable.
The intermediate frequency cable is configured to transmit intermediate frequency signals, low frequency control signals, and power. The low frequency control signals are configured to provide the communication between the indoor unit and the outdoor unit, and the ASK or FSK modulation technologies are generally used to perform modulations and demodulations.
On one side that transmits the control signals, it is relatively simple to perform the modulation of the control signals. While on the other side that receives the control signals, it is relatively complicated to demodulate the modulated control signals. The reason mainly lies in that coupling of DC power tends to introduce broadband switch noises, and these noises are very difficult to be filtered completely by filters of receiving side equipments, and tend to form several interference pulses after passing through a detection circuit. At this time, if a threshold value of a decision circuit happens to be set somewhat low, or the signal amplitude is relatively small, error codes tend to occur.
Referring to FIGS. 1 and 2, a conceptual circuit block diagram of an ASK or FSK demodulation device is shown respectively. An ASK or FSK demodulation device commonly used in the industry is composed of a bandpass filter 4, an envelope detection circuit 5, a low-pass filter circuit 6, and a sampling decision circuit 7 connected in turn. The sampling decision circuit 7 inputs a timing pulse 2 for sampling. The two are different in that, the ASK demodulation circuit inputs a signal 1 that has only one frequency, while an FSK signal 3 is modulated with two frequencies (usually not separated very far). The FSK demodulation circuit needs to filter the signals of the two different frequencies respectively. Therefore, the FSK demodulation circuit has loops respectively configured to demodulate the signals of two different frequencies.
In microwave communication, the low frequency ASK signal 1 is often transmitted with a DC power, and wideband switch noises generally exist in the power. In the circuit shown in FIG. 1, the bandpass filter 4 cannot filter completely the switch noises, and thus the switch noises tend to form pulse interferences after passing through the envelope detection circuit 5. When a threshold of the sampling decision circuit 7 is set relatively small, the output data tends to have error codes. But when small signals are considered, the threshold cannot be set too high, which causes a weak pulse interference resistance capability of the demodulation circuit.
The principle of the FSK demodulation circuit shown in FIG. 2 is similar to that of the ASK demodulation circuit shown in FIG. 1. The two are different in that, because the FSK modulation signal 3 has signals of two frequencies, the FSK demodulation circuit has to perform bandpass filtering, envelope detections, low-pass filtering, and sampling decisions respectively on the signals of two different frequencies. Therefore, the FSK demodulation circuit shown in FIG. 2 has the same shortcomings as those of the ASK demodulation circuit shown in FIG. 1. In addition, because the two frequencies in the FSK modulation signal are closer, the design of the filter becomes more difficult consequently.
Moreover, the ASK demodulation circuit and the FSK demodulation circuit are only suitable for ASK demodulations or FSK demodulations respectively. Users of the digital microwave system may purchase different indoor units or outdoor units from different equipment manufacturers respectively. If the indoor units use the ASK signals, and the outdoor units use the FSK signals, the users of the digital microwave system or the equipment manufacturers need to make corresponding reconstruction to the demodulation circuits of the indoor units or the outdoor units, thus making the two compatible with each other. In existing systems, corresponding hardware circuit designs need to be changed when there are changes to the modulation schemes of the equipments, which is quite inconvenient.